In general, an electrophotographic method in a Plain Paper Copy (PPC) copying machine or a printer in which a toner image formed on a photoreceptor is transferred onto a recording paper is performed by a procedure as follows. First, an electrostatic latent image is formed on the photoreceptor. Next, after the latent image is developed by using a toner and the toner image is transferred onto a sheet to be fixed such as a paper, the toner image is fixed by heating with a heat roll or a film. In this method, since fixing is performed under heating in a state in which the toner on the sheet to be fixed directly comes into contact with a heat roll or a film, fixing is promptly performed and a thermal efficiency is extremely excellent. Therefore, a fixing efficiency is very good.
However, in this heat fixing system, though having an excellent thermal efficiency, since the toner comes into contact with the surface of a heat roll or a film in a melted state, there is a problem which is a so-called offset phenomenon, in which a transfer paper or the like which is sent next is contaminated with the toner which is shifted onto the heat roll at this time.
In addition, a load applied to the toner has been becoming bigger in accordance with high printing speed. There is a problem in which an image defect easily occurs due to the toner being destroyed because of this load, and thus the charged state of the toner being changed. Therefore, high durability is becoming necessary for the toner. However, there is a problem in which the productivity of the toner deteriorates as the strength of a resin increases in order to improve the durability.
Furthermore, since the calorific value which is given to the toner when fixed is reduced in accordance with high printing speed, higher fixing performance is beginning to be demanded.
In order to obtain a toner excellent in a balance among fixing properties, offset resistance, durability, and toner productivity, there has been known a binder resin in which a resin having a high molecular weight and a resin having a low molecular weight are mixed to be used and a high molecular weight part is crosslinked (for example, Patent Documents 1 to 7).